When a finish goes wrong, the booth filters get blamed first. Sometimes that's right. But fisheyes, poor adhesion, cloudy clear and grit that seems to come from nowhere often travel in through the air hose, not the ceiling. Your compressor is pulling in shop air, squeezing it, heating it and pushing it through steel pipe, and every step adds something you don't want on a panel.
This guide traces each contaminant to its source, matches it to the defect it causes, and lays out a filtration setup that stops it. Examples use the RTi product line, which we carry as an authorized dealer.
The three contaminants and where they come from
Water
Every cubic foot of air your compressor draws in carries water vapor. Compressing it concentrates that moisture, and as the hot compressed air cools in the tank and lines, much of it condenses into liquid. Whatever stays as vapor can still condense later in a cold line or hose, or at the gun.
Shows up as: spitting at the gun, blushing and cloudiness in clear, slow flash with waterborne, and poor adhesion.
Oil
Oil-lubricated compressors put a small amount of oil into the air stream as fine droplets (aerosol) and as vapor. Even oil-free compressors can pass along hydrocarbons drawn in from the shop. Oil aerosol is too small to see and oil vapor passes straight through ordinary particulate filters.
Shows up as: fisheyes and cratering, poor adhesion and finishes that won't lay down evenly.
Dirt and particulate
Rust and scale from steel piping, dust pulled in at the compressor intake and debris from fittings all ride along in the air. Wet lines make it worse, because moisture speeds up rust inside the pipe.
Shows up as: nibs and grit that keep coming back even with fresh booth filters. If you're chasing dirt in the finish, rule out the booth first with our guide to diagnosing dirt nibs, then look at the air line.
How filtration stages divide the work
No single filter removes everything. A good paint air setup works in layers, with each stage protecting the next.
| Stage | Removes | RTi example |
|---|---|---|
| Mainline separator / particulate filter at the compressor | Liquid water, dirt and scale to 1 micron | EX108 Eliminex® (150 SCFM, 1") |
| Mainline coalescer | Oil aerosol and fine particulate to 0.01 micron | C108 coalescer, or the EX208 combination unit |
| Refrigerated dryer | Bulk water vapor for the whole shop | RDNC non-cycling refrigerated dryers |
| Point-of-use filter for tools and prep | Water, dirt and oil vapor at the station | STEALTH 6950B / 6960B / 6060B |
| Point-of-use desiccant dryer at the booth | Remaining moisture to -40°F dew point, oil vapor, fine particulate | Eliminator® II or Perfect Air® |
The mainline stages protect the whole shop and take the heavy load off everything downstream. The point-of-use stages finish the job right where the air is used. For the paint gun itself, that last stage is what decides the finish. We cover choosing it in why waterborne paint needs truly dry air.
Why filter design matters: RTi's Inverse-Flow®
Most filters push air through media and hope the contaminants stay put. RTi's patented Inverse-Flow® Technology separates them before the air reaches the media:
- Centrifugal spinning. Incoming air spins downward, flinging heavy water, dirt and oil droplets outward toward the drain.
- Stainless steel mesh. Pre-cleaned air passes through interwoven stainless mesh that holds aerosols and droplets, which fall to the drain.
- The 180° turn. The air reverses and flows upward. Heavier contaminants can't make the turn, so they separate out instead of being carried back into the stream.
- Final media. A cotton, polyester and stainless element captures remaining fine water droplets to 1.0 micron.
Because contaminants drop out before the final media, the element lasts longer and the filter doesn't re-release what it caught.
Five habits that keep air clean between filter changes
- Drain daily. RTi's maintenance guide says to open manual drains at least once a day. A full bowl puts liquid straight back into the line.
- Filter close to the point of use. Air that cools after the last filter can condense again. Put the final stage near the gun or tool.
- Mount filters vertical with the drain down, airflow following the arrow on the head.
- Keep units from freezing and stay within the rated pressure and temperature (RTi: 150 PSIG for polycarbonate bowls or units with differential pressure indicators, 125°F max).
- Change elements on schedule, not when the finish tells you. See the element change schedule for every RTi unit.
Frequently asked questions
What causes fisheyes from compressed air?
Fisheyes from the air supply are almost always caused by oil, carried from the compressor as fine aerosol or vapor. A coalescing filter removes oil aerosol to 0.01 micron, and an activated carbon stage removes oil vapor.
Why do I still get water at the gun with a refrigerated dryer?
A refrigerated dryer leaves air with a dew point in the high 30s °F. If that air cools further in a cold line or hose, the remaining moisture condenses. A point-of-use desiccant dryer at the booth drops the dew point to -40°F, so nothing is left to condense.
How often should compressed air filters be drained?
RTi's general maintenance guide says manual drains should be opened at least daily. Units with automatic float or electronic drains handle this on their own but should still be checked.
Do I need filters at the compressor and at the booth?
Yes, ideally. Mainline filters and dryers protect the whole shop and take the bulk load. Point-of-use filters and dryers clean up what's left right where the air is used, which is what protects the finish.
Trace the problem, then fix it at the source
If you know which defect you're fighting, you know which stage is missing. Send us a description of your setup, from compressor to booth, and we'll help you fill the gap with the right RTi unit. RTi publishes detailed literature for every product at rti-pbe.com.
Clean Air Starts at the Compressor
From mainline filters to point-of-use dryers, we carry RTi's full compressed-air line as an authorized dealer. RTi orders over $1,200 ship free.
See RTi filters and dryers →
Tell us about your air setup →
Choosing a dryer for waterborne →